refactor: replace legacy Config with Settings

Remove legacy src/config.rs and migrate all application consumers to
the nested Settings model. Baseliner, metrics, pingers, ratecontroller,
reflector selector, and main now use Settings. Derived min rates become
methods on NetworkSettings rather than stored legacy fields.
MeasurementType and ObservabilityProtocol move into the settings module.

src/pinger_icmp_ts.rs

@@ -5,8 +5,8 @@
 //
 // SPDX-License-Identifier: MPL-2.0
 
-use crate::config::MeasurementType;
 use crate::pinger::{PingError, PingListener, PingReply, PingSender};
+use crate::settings::MeasurementType;
 use crate::time::Time;
 use icmp_socket2::Icmpv4Message;
 use icmp_socket2::Icmpv4Packet;